1. Field of the Invention
The present invention relates to a mobile communication method in which a mobile station performs location registration to one of exchanges managing a location registration area where the mobile station is located, in a mobile communication system where a plurality of exchanges manage one location registration area and a radio network controller is connected to the plurality of exchanges.
The present invention also relates to a mobile station and a radio network controller which can be used in the mobile communication method.
2. Description of the Related Art
Conventionally, a mobile communication system in which one location registration area is managed by a plurality of exchanges so that processing load in each exchange can be dispersed is known. Referring to FIGS. 1A and 1B, this type of mobile communication system will be explained.
In this mobile communication system, as shown in FIG. 1A, a radio network controller RNC is connected to a first exchange for a packet switched service SGSN (Serving GPRS Support Node) #1 and a second exchange for a packet switched service SGSN #2, which manage a same routing area (location registration area) RA #1, via predetermined interfaces (e.g. Iu-flex interfaces).
When receiving a location registration request from a mobile station UE in the routing area RA #1, the radio network controller RNC is configured to determine an SGSN to which the mobile station UE performs the location registration, in accordance with the load status in each SGSN.
The radio network controller RNC is configured to transfer the location registration request from the mobile station UE to the determined SGSN.
In FIG. 1A, a mobile station UE #1 performs the location registration to the first SGSN #1 in the routing area RA #1, and transmits signals for each type of packet switched service via the first SGSN #1.
On the other hand, a mobile station UE #2, which is located in the same routing area RA #1, performs the location registration to the second SGSN #2, and transmits signals for each type of packet switched service via the second SGSN #2.
However, in the conventional mobile communication system, there has been a problem in that, in the case where congestion or failure occurs in one of a plurality of SGSNs managing a same routing area RA #1, not only mobile stations UE registered in the SGSN but also mobile stations UE registered in other SGSNs fail to transmit signals for each type of packet switched service to the registered SGSN, that is, those mobile stations UE cannot use packet switched service in that situation.
Specifically, as shown in FIG. 1B, when congestion or failure occurs in the first SGSN #1, in step S1, the first SGSN #1 transmits a state signal to the radio network controller RNC, so as to notify the occurrence of the congestion or the failure, in step S2.
In step S3, the radio network controller RNC broadcasts an access restriction signal, which restricts accesses to the packet switched service domain, to mobile stations UE in the routing area RA #1.
Here, in the conventional mobile communication system, the radio network controller RNC cannot identify the first SGSN #1 where the congestion or the failure occurs, in the access restriction signal.
Therefore, in step S4, the mobile station UE #1 stops transmitting signals for each type of packet switched service addressed to the first SGSN #1, in accordance with the received access restriction signal.
And, in step S5, the mobile station UE #2 also stops transmitting the signal for each type of packet switched service addressed to the second SGSN #2, in accordance with the received access restriction signal.
Further, in step S6, a mobile station UE #3, which has newly moved into the routing area RA #1 from another routing area, cannot transmit a location registration request in routing area PA #1 even though the second SGSN #2 is available.